The present invention relates to an illuminating device having optical light guide in the form of a fibre bundle which may be used as an endoscope.
Generally, optical light guide fibres are made from glass or plastic material by crucible, acid melting method, rod, or thermal fusion method. These fibres are bundled, end rings are fitted at both ends of the bundle, and both ends are suitably ground to form a fibre bundle of flexible, semi-rigid or rigid conduit.
Such fibre bundles are used in light communication systems, illuminating devices sensor heads, light image transportation systems, illuminating devices light and safety anti-explosion devices. In any case, it is important to obtain a low loss and good light transporting fibre bundle.
One proposal to achieve this result is described in Japanese Laid Open Patent Application No. 19761/1979. This application attempts to decrease Fresnel reflection on both input and output end surfaces of the fibre bundle. To this end, a reflection decreasing layer e.g. MgF.sub.2 is applied on a glass base plate having substantially the same refractive index as that of the fibre bundle and the glass base plates are adhered on the input and output end surfaces of the fibre bundle by a transparent adhesive having substantially the same refractive index as that of the fibre bundle.
While this structure decreases Fresnel reflection loss on both the end surfaces of the fibre bundle, it does not uniformly distribute input light to the input surface of the fibre bundle.
In the field of endoscopy, high accuracy and high capacity are desired and especially a high level of light transmission is desired. In conventional endoscopes, to improve light transmission, a strong light source, e.g. small xenon lamp of high luminance, is used and also a condenser lens is inserted to concentrate the high output of the lamp in narrow end surface of the fibre bundle efficiently.
However, this structure may cause other problems such as a shortened life of the lamp and a burning of the end surface of the fibre bundle by highly concentrated high luminance light. Consequently, the most intimate problem of such illuminating device is to concentrate the illuminating light from the light source uniformly and with least light loss to the incidence end surface of the fibre bundle.